The invention relates to a low-pressure mercury vapour discharge lamp having a very satisfactory colour rendition, a colour temperature of the emitted white light in the range of 2300 to 3300 K. and a colour point on or near the Planckian curve and provided with a gas-tight radiation-transparent envelope containing mercury and rare gas and with a luminescent layer containing a luminescent halophosphate and a luminescent material activated by bivalent europium.
The expression "a very satisfactory colour rendition" is to be understood to mean in the present description and the appended claims that the average colour rendering index R(a,8) (average value of the rendering indices of eight test colours as defined by the Commission Internationale d'Eclairage: Publication CIE, No. 13.2 (TC-3.2), 1974) has a value of at least 85.
The colour of visible radiation is characterized by the colour coordinates (x,y) determining the colour point in the colour triangle (see Publication CIE, No. 15 (E-1.3.1), 1971). Lamps for general illumination purposes should emit light which can be considered to be white. White radiation is found in the colour triangle at colour points located on the Planckian curve. This curve, which is also designated as the curve of the black body radiators and which will be denoted hereinafter as the curve P, comprises the colour points of the radiation emitted by a completely black body at different temperatures (the so-called colour temperature). A given colour temperature is allotted not only to a given point on the curve P, but also to radiation having colour coordinates located on a line intersecting the curve P at that point (see the said Publication CIE, No. 15). If this radiation has a colour point near the curve P, this radiation is also considered as white light having this given colour temperature. In the present description and the appended claims, the expression "a colour point near the curve P" is to be understood to mean that the distance of the colour point from the point on the curve P having the same colour temperature is at most 20 MPCD. MPCD (Minimum Perceptible Colour Difference) is the unit of colour difference (see the Publication of J. J. Rennilson in Optical Spectra, October 1980, page 63).
A large number of embodiments of low-pressure mercury vapour discharge lamps which have been known for many years and are frequently used contain a luminescent material chosen from the group of the alkaline earth metal halophosphates activated by Sb.sup.3+ and Mn.sup.2+. These lamps have the advantage that they are inexpensive and emit a satisfactorily high luminous flux. A great disadvantage of these lamps, however, is that their colour rendition leaves much to be desired. They generally have R(a,8) values of the order of 50 to 60 and only in lamps at a high colour temperature (for example 5000 K.) is a value of R(z,8) of approximately 75 reached, which is not yet considered to be a satisfactory colour rendition.
Lamps with which a very high colour rendition is reached have been known for a long time. These lamps are provided with special luminescent materials, i.e. a tin-activated red-luminescing material on the basis of a strontium orthophosphate most frequently combined with a blue-emitting halophosphate activated by Sb.sup.3+, in particular such a strontium halophosphate. The strontium orthophosphate luminesces in a very wide band which extends into the deep red. These known lamps have the disadvantage inherent in the use of the strontium orthophosphate of a comparatively small luminous flux and of a poor maintenance of the luminous flux during the life of the lamp. It has been found that, because of the latter disadvantage, this strontium orthophosphate is of little practical use when subjected to a higher load resulting from mercury discharge radiation.
A lamp of the kind described in the opening paragraph is known from German Patent Application No. 2,848,726. This lamp which has a very satisfactory colour rendition contains, like the aforementioned lamp type, a red-luminescing tin-activated strontium orthophosphate and further a borate-phosphate activated by bivalent europium, which has an emission band with a maximum at approximately 480 nm and a half-value width of approximately 85 nm. Preferably, a luminescent alkaline earth metal halophosphate is further used in the luminescent layer of this lamp. Due to the use of the luminescent strontium orthophosphate, this known lamp again has the disadvantage of a comparatively low luminous flux and in particular of a poor maintenance of the luminous flux during the life of the lamp. The known lamp further has the disadvantage that a very satisfactory colour rendition is reached only at colour temperature above approximately 3500 K. Embodiments of the known lamp at very low colour temperatures (below 3000 K.) are not possible.